1. Field of the Invention
The present invention relates to a method and apparatus for detecting the location of magnetic marks on a wireline cable and for accurately making depth measurements during wireline logging.
2. Description of Related Art
One method in common use for providing a wireline depth measurement involves the use of encoder wheels which frictionally engage the cable or wireline to detect its movement. One such system is described in U.S. Pat. No. 4,179,817 (incorporated by reference). In this type of system, two encoder wheels are used for redundancy. As the cable moves, the wheels turn and, by counting the revolutions of the wheel, the distance the cable travels going into and out of the borehole can be measured. Such encoder wheels give an approximate estimate of cable distance travel or depth of the tools in the borehole.
U.S. Pat. No. 4,924,596 (incorporated by reference) describes a method for correcting slippage errors of such encoder wheels during wireline depth measurements. That is, wireline depth measurements are very important and U.S. Pat. No. 4,924,596 represents a method for more accurately obtaining depth measurements.
It is known to apply or impress magnetic marks to wireline cable at regular, precise intervals, for example 100 feet. See, U.S. Pat. No. 3,566,478 (incorporated by reference). The encoder wheel approximate depth measurements can be correlated to the magnetic mark location to give better depth assessments. The armor of a wireline cable is magnetized at periodic depths, either by a hand magnet or one of several electromagnetic marking coils. This magnetic mark is usually detected through a Hall Effect sensor. A mark signal looks significantly different depending on the method with which it was impressed. FIGS. 2A-B show the mark energy strength versus depth of various marking techniques. Note especially the large negative dip 30 on one side of the main peak on the AMD mark in FIG. 2A, and the substantial minor positive peak on the IDW mark in FIG. 2B.
The mark signal amplitude degrades as cable is moved through the well. This degradation is not uniform across the cable because the shallow end of the cable goes in and out of the well more often than the deeper end. After the mark signal has degraded so much that the mark can no longer be detected from noise, the cable marks must be refreshed (re-magnetized).
In the earliest mark detection systems, an analog peak detector was used to find energy peaks above a certain threshold--the location of such a peak was declared the location of a mark. Later systems implemented the same algorithm digitally by sampling the Hall Effect sensor at periodic time intervals, then taking the peak (above a threshold) of these samples.
Current approaches for detecting magnetic marks on a wireline cable have major problems. The most important difficulty is finding an appropriate threshold; the threshold must be above the noise, yet below the peak. Unfortunately, the amplitude of the peak changes as the mark slowly degrades and is consequently refreshed. Worse, the "false peak" part of an IDW mark (FIG. 2B) must be considered part of the noise, leading to a signal-noise ratio of sometimes less than 2. If the threshold is too high, the mark will be missed completely, yet if it is too low, two marks will be detected (or one mark if the second peak is ignored, but the mark will be found at different depths depending on whether the wireline cable is moving uphole or downhole). A wireline logging field engineer attempts to manually determine an appropriate threshold. The time-based sampling of the digital algorithm also causes a problem. Magnetic marks are usually about ten inches long; if a cable is moving fast enough, it is conceivable that a mark may be missed entirely. Cable speed affects the spectral content of these time-based samples, making the development of signal processing algorithms for magnetic mark detection difficult.
Thus, it would be a significant advance if a method and apparatus were devised for efficiently and accurately detecting the location of the magnetic marks on a wireline cable and integrating this magnetic mark detection with encoder wheel measurements for accurate depth determination. In the present application, depth of the logging tools within the borehole can be inferred from cable travel--therefore, "distance" and "depth" are sometimes used interchangeably. The signal strength detected by a Hall Effect sensor (e.g., FIG. 2) is sometimes referred to as a "signal value" or "signal level."